Recently, the scanning tunneling microscope (STM) has been developed for resolving the composition of a sample surface down to the atomic level.
The measuring principle of the STM is described below.
Above the sample surface to be measured, a probe made of an extremely sharp pointed metal needle is brought within a distance of about 10 angstroms, and a bias voltage of about 10 mV to 2 V is applied between sample and probe, with a tunneling current of several nA then flowing. The tunneling current is very sensitive to the distance between sample and probe; therefore, this distance may be kept constant by employing feedback control.
The probe is attached to an actuator composed of piezo elements capable of inching movement in three axial directions, X, Y and Z. When a raster scan is directed by moving sample and probe relatively in the planar X-Y direction, the distance between sample and probe is kept constant by feedback control to the Z direction. Incidentally, the change in voltage applied to the position control of the Z direction directly expresses the asperities of the sample surface.
Such STM technology is producing peripheral technologies like the atomic force microscope (AFM), near field optical microscope (NFOM) and superprecision processing machines capable of manipulating atoms and molecules. These technologies commonly employ the method of scanning the sample surface while maintaining the distance between sample and probe at the atomic level. Including the observation apparatuses, the above-noted apparatuses are commonly called near field microscopes (NFM) or scanning probe microscopes (SPM).
While maintaining a distance between probe and sample at an atomic level, the sample surface can be scanned by thoroughly eliminating mechanical vibrations affecting distance fluctuations. Thus, the foremost consideration of a positioning device of the invention should be the elimination of mechanical vibrations.
Moreover, when the STM is used to analyze a semiconductor element, superlattice quantum effect device or the like, the positioning mechanism is necessary to locate a specific position of the sample. Furthermore, for a superprecision processing machine utilizing STM technology, a positioning device of high accuracy is needed to control X-Y position coordinates.
As an example of a simple positioning function of a STM used in an air environment, without installation of a specific positioning device, the probe head with the scanning probe was manually moved while observing sample and scanning probe from an oblique direction by an optical microscope.
Moreover, as an example of a positioning device for a STM used in a vacuum, the sample was positioned by a worm-type driving device while observing sample and scanning probe from an oblique direction with a scanning electrochemical microscope (SEM).
Although in the conventional STM, however high in precision the absolute positioning function of the X-Y position coordinates of sample and scanning probe may be, so far as the mechanical vibrations cannot be eliminated completely, it is insufficient for accurate X-Y positioning.
When scanning at a distance of an atomic level from the scanning probe to the sample, mechanical vibrations affecting the fluctuations of distance must be thoroughly eliminated. In the conventional positioning device, however, it is difficult to satisfy both vibration control and accurate X-Y positioning.